The present invention relates to high temperature permanent magnet materials, and more particularly to permanent magnets which have improved magnetic properties at a desired operating temperature.
Permanent magnets containing one or more rare earth elements and transition elements are well known for use in a variety of applications. For example, magnets have been used in motors and generators for aircraft and spacecraft systems. Magnets have also been widely used in actuators, inductors, inverters, magnetic bearings, and regulators for flight control surfaces and other aircraft components. These applications require the magnets to operate at temperatures up to about 300xc2x0 C.
In recent years, the need has increased for magnetic and electromagnetic materials capable of reliable operation at higher temperatures of from 300xc2x0 C. to 600xc2x0 C. For example, the MEA (More Electric Aircraft) initiative has stimulated the development of an Integrated Power Unit (IPU) which utilizes a high-speed, direct-coupled starter/generator and magnetic bearings integrated onto the rotor of a single-shaft gas turbine aircraft engine which permits direct coupling to the turbine shaft, thereby eliminating all gearing and lubrication found in current military and commercial aircraft power units. However, the operating temperature of magnetic materials for such an application is higher than 400xc2x0 C. Other high temperature applications include replacement of hydraulic-mechanical components in aircraft with permanent magnets. Accordingly, magnetic materials capable of operating at temperatures as high as 400xc2x0C. and above are needed for such applications.
Currently, Sm(CowFevCuxZry)z or RE2TM,17 (where RE represents rare earth metals and TM represents transition metals) permanent magnet materials have been demonstrated as the best magnets for elevated temperature applications. These magnets have satisfied many applications at temperatures up to 300xc2x0 C. However, such magnets have typically been unable to retain their magnetic properties at elevated temperatures greater than 300xc2x0 C. For example, the intrinsic coercivity (iHc) of such magnets has been found to drop substantially at temperatures of 300xc2x0 C. or greater. More importantly, the extrinsic demagnetization curve for such magnets is not straight, and linear extrinsic demagnetization curves are imperative for dynamic applications. In order to maintain stability at high temperatures, magnets must maintain a high iHc and a low temperature coefficient of iHc as well as a linear extrinsic demagnetization curve.
Accordingly, there is still a need in the art for a permanent magnet material which is capable of operating at temperatures higher than 300xc2x0 C., which exhibits a high iHc and a low temperature coefficient of iHc, and which exhibits a linear extrinsic demagnetization curve at high temperatures.
As additional background information, the early development of rare earth magnet alloy systems is discussed in the following papers:
K. Strnat and W. Ostertag, xe2x80x9cProgram for an in-house investigation of the yttrium-cobalt alloy systemxe2x80x9d, Technical Memorandum, May 64-4, Projects 7367 and 7360, AFML, Wright-Patterson AFB, Ohio, March, (1964)
K. Strnat and G. Hoffer, xe2x80x9cYCo5 - A promising New Permanent Magnet Materialxe2x80x9d, USAF Tech. Doc. Rept., Materials Laboratory, WPAFB AFML-TR-65-446, May (1966).
G. Hoffer and K. Strnat, xe2x80x9cMagnetiocrystalline Anisotropy of YCO5 and Y2Co17xe2x80x9d, IEEE Trans. Magn., Mag-2, 487, Sept., (1966).
K. Strnat, G. Hoff, J. Olson, W. Ostertag, and J. Becker, xe2x80x9cA family of new cobalt-base permanent magnet materialxe2x80x9d, J. Appl. Phys. 38 1001, (1967)
D. Das, xe2x80x9cTwenty million energy product samarium-cobalt magnetxe2x80x9d, IEEE Trans., Magn. Mag-5, 214, (1969)
M. Benz and D. Martin, xe2x80x9cCobalt-samarium permanent magnets prepared by liquid phase sinteringxe2x80x9d, Appl. Phys. Lett., 17 176 (1970)
RE2TM17 type magnets were initiated from the investigation of R2(Co, Fe)17 alloy by A. E. Ray and K. J. Strnat in 1972. However, numerous attempts to develop high iHc in these stoichiometric 2:17 alloys were generally unsuccessful and attention was then focused on Sm(Co.85Cu.15)6.8 (Nagel et al., 1975) and Sm(Co.85Fe.05Cu.10)8 (Tawara et al., 1976) with Br=10-11 kG, Hc=4-6 kOe, and (BH)max=26 MGOe. Sm(Co.68Fe.28Cu.1Zr.01)7.4 with 30 MGOe was achieved in 1977 (Ojima et al., 1977). Research and development in the 1970""s resulted in RE2TM17 type magnets with high energy product, where RE represents rare earth metals, such as Sm, Pr, Gd, Ho, Er, Ce, Y, Nd, and TM represents transition metals such as: Co, Fe, Cu, Zr, Hf, Ti, Mn, Nb, Mo, W, and other transition metals. Particularly preferred high performance magnets for the applications noted above are RE=Sm, Gd, Dy and TM=Co, Fe,Cu, and Zr, having the crystal structure of Sm2Co17. Most RE-TM magnets can be used at 250xc2x0 C., and some of these magnets can perform well up to 330xc2x0 C.
Some of these magnets are described in U.S. Pat. Nos. 4,210,471; 4,213,803; 4,284,440; 4,289,549; 4,497,672; 4,536,233; 4,565,587, 4,746,378, and 5,781,843. See also U.S. Pat. Nos. 3,748,193, 3,947,295; 3,970,484; 3,977,917; 4,172,717; 4,211,585; 4,221,613; 4,375,996; 4,382,061 and 4,578,125.
Publications relating to RE2TM17 type magnets are listed below:
A. E. Ray and K. J. Strnat, IEEE Trans. Magn., Mag-8, 518, 1972
Nagel, Perry and Menth, IEEE Trans. Magn. Mag-11, 1423, 1975
Tawara and Strnat, xe2x80x9cRare earth Cobalt permanent magnets near the 2:17 compositionxe2x80x9d, IEEE Trans. Magn. Mag-12, 954, 1976
Ojima, Tomizawa, Yoneyama, and Hori, xe2x80x9cMagnetic properties of a new type of rare earth magnets Sm2(Co,Cu,Fe,M)17,xe2x80x9d IEEE Trans. Magn. Mag-13, 1317, 1977
A. E. Ray, xe2x80x9cThe development of high energy product permanent magnets from 2:17 RE-TM alloysxe2x80x9d, IEEE Trans, Ma-20, 1615, (1984)
Marlin S. Walmer, xe2x80x9cA comparison of temperature compensation in SmCo5 and RE2TM17 as measured in a permeameter, a traveling wave tube and an inertial device over the temperature range of xe2x88x9260xc2x0 to 200xc2x0 C.xe2x80x9d, Proceedings of the 9th International workshop on rare earth magnets and their applications, Bad Soden, Germany, 131-140 (1987)
H. F. Mildrum and K. D. Wong, xe2x80x9cStability and temperature cycling behavior of RE-Co magnetsxe2x80x9d, Proceedings of the 9th International workshop on rare earth magnets and their applications, Bad Soden, Germany, 35-54 (1987)
J. Fidler, et al., xe2x80x9cAnalytical Electron microscope study of high and low coercivity SmCo 2:17 magnetsxe2x80x9d, Mat. Res. Sol. Sym. Proc. 96, 1987
Popov et al., xe2x80x9cInference of copper concentration on the magnetic properties and structure of alloysxe2x80x9d, Phys. Met. Metall., 60 (2), 18-27, (1990)
A. E. Ray and S. Liu, xe2x80x9cRecent progress in 2:17 type permanent magnetsxe2x80x9d, J. Material Engineering and Performance, 1, 183-192, (1992)
Extrinsic demagnetization curves for prior art Sm-TM magnet materials are set forth in FIG. 1 which shows that linear extrinsic demagnetization curves existed up to about 330xc2x0 C. The curves become non-linear above 330xc2x0 C.
FIG. 2 illustrates the recoil process for a magnet with a nonlinear extrinsic demagnetization curve, when the demagnetization force drives the magnet past the xe2x80x9ckneexe2x80x9d of the curve and back to zero magnetic strength.
Further work has been done on RE-TM magnets for use at temperatures above 300xc2x0 C. References related to these high temperature RE-TM magnets are listed below:
Marlin S. Walmer and Michael H. Walmer, xe2x80x9cKnee formation of high Co content 2:17 magnets for MMC high temperature applicationsxe2x80x9d, EEC internal report, May, 1995
S. Liu and E. P. Hoffman, xe2x80x9cApplication-oriented characterization of Sm2(Co,Fe,Cu,Zr)17 permanent magnets,xe2x80x9d IEEE Trans. Magn., 32, 5091, (1996).
B. M. Ma, Y. L. Liang, J. Patel, D. Scott, and C. O. Bounds, xe2x80x9cThe effect of Fe content on the temperature dependent magnetic properties of Sm(Co,Fe,Cu,Zr)z and SmCo5 sintered magnets at 450xc2x0 C.,xe2x80x9d IEEE Trans. Magn., 32,4377 (1996).
S. Liu, G. P. Hoffman, and J. R. Brown, xe2x80x9cLong-term aging of Sm2(Co,Fe,Cu,Zr)17 permanent magnets at 300 and 400xc2x0 C.,xe2x80x9d IEEE Trans. Magn., 33, 3859 (1997)
A. S. Kim, J. Appl. Phys. 81, 5609 (1997).
C. H. Chen, M. S. Walmer, M. H. Walmer, S. Liu, E. Kuhl, G. Simon, xe2x80x9cSm2(Co,Fe,Cu,Zr)17 magnets for use at temperature 3 400xc2x0 C.xe2x80x9d, J. Appl Phys. 83 (11), 6706 (1998)
A. S. Kim, xe2x80x9cHigh temperature stability of SmTM magnets,xe2x80x9d J. Appl. Phys., 83 (11), 6715 (1998)
M. S. Walmer, C. H. Chen, M. H. Walmer, S. Liu, G. E. Kuhl, G. K. Simon, xe2x80x9cUse of heavy rare earth element Gd in RECo5 and RE2TM17 magnets for high temperature applicationsxe2x80x9d, Proc. 15th Int. Workshop on Rare Earth Permanent Magnets and Their Applications, p. 689, (1998)
A. S. Kim, U.S. Pat. No.: 5,772,796 (1998)
J. F. Liu, Y. Zhang, Y. Ding, D. Dimtar, F. Zhang, and G. C. Hadjipanayis, xe2x80x9cRare earth permanent magnets for high temperature applicationsxe2x80x9d, Proc. 15th Int. Workshop on Rare Earth Permanent Magnets and Their Applications, p. 607, (1998)
Christina H. Chen, Marlin S. Walmer, Michael H. Walmer, Wei Gong, and Bao-Min Ma, xe2x80x9cThe relationship of thermal expansion to magnetocrystalline anisotropy, spontaneous magnetization and Tc for permanent magnetsxe2x80x9d, J. Appl. Phys., 85(8), 5669 (1999)
J .F. Liu,, Y. Zhang, D. Dimitar, and G. C. Hadjipanayis, xe2x80x9cMicrostructure and high temperature magnetic properties of Sm(Co,Cu,Fe,Zr)z (z=6.7-9.1) permanent magnetsxe2x80x9d, J. Appl. Phys. 85(5), 2800 (1999)
Sam Liu, Jie Yang, George Doyle, G. Edward Kuhl, Christina Chen, Marlin Walmer, Michael Walmer, and Gerard Simon, xe2x80x9cNew sintered high temperature Sm-Co based permanent magnet materialsxe2x80x9d, IEEE Trans. Magn.35, 3325 (1999)
Sam Liu and G. Edward Kuhl, xe2x80x9cTemperature coefficients of Rare earth permanent magnetsxe2x80x9d, IEEE Trans. Magn. 35, 3371 (1999)
Christina H. Chen, Marlin S. Walmer, Michael H. Walmer, Sam. Liu, E. Kuhl, Geared K. Simon, xe2x80x9cNew Sm-TM magnetic materials for application up to 550xc2x0 C.xe2x80x9d, 1999 Spring meeting, MRS Symposia Proceedings, to be published, (1999).
Christina H. Chen, Marlin S. Walmer, Michael H. Walmer, Jinfang Liu, Sam Liu, E. G. Kuhl, xe2x80x9cMagnetic pinning strength for the new Sm-TM magnetic materials for use up to 550xc2x0 C.xe2x80x9d, 44th conference in 1999, to be published in J. Appl. Phys., April, (2000)
Sam Liu, Jie Yang, George Doyle, Gregory Potts, and G. Edward Kuhl C. H. Chen, M. S. Walmer, M. H. Walmer, xe2x80x9cAbnormal temperature dependence of intrinsic coercivity in sintered Sm-Co based permanent magnetsxe2x80x9d, 44th MMM conference in 1999, to be published in J. Appl. Phys., April, (2000)
Marlin S. Walmer, Christina H. Chen, Michael H. Walmer, Sam Liu, E. G. Kuhl, xe2x80x9cThermal stability at 300-550xc2x0 C. for a new series of Sm2TM17 materials with maximum use temperature up to 550xc2x0 C.xe2x80x9d, Intermag 2000, to be published in IEEE Trans. Mag. (2000)
Sam Liu, Gregory Potts, George Doyle, Jie Yang, and G. Edward Kuhl, C. H. Chen, M. S. Walmer, M. H. Walmer, xe2x80x9cEffect of z value on high temperature performance of Sm(Co,Fe,Cu,Zr)z with z=6.5-7.3xe2x80x9d, Intermag 2000, to be published in IEEE Trans. Mag. (2000)
The present invention provides a new class of permanent magnets which have optimum magnetic properties at specific high operating temperatures. The permanent magnets show high resistance to thermal demagnetization and exhibit linear extrinsic demagnetization curves at elevated temperatures up to 700xc2x0 C.
According to one aspect of the present invention, a permanent magnet is provided which is represented by the general formula RE(CowFevCuxTy)z, where RE is a rare earth element selected from the group consisting of Sm, Gd, Pr, Nd, Dy, Ce, Ho, Er, La, Y, Tb, and mixtures thereof, and T is a transition metal selected from the group consisting of Zr, Hf, Ti, Mn, Cr, Nb, Mo, W, V, Ni, Ta, and mixtures thereof, wherein the magnet exhibits a substantially linear extrinsic demagnetization curve at a maximum operating temperature TM up to 700xc2x0 C. xe2x80x9cTMxe2x80x9d is defined in this invention as the maximum operating temperature where a linear extrinsic demagnetization curve can exist for the magnet. Preferably, the substantially linear extrinsic demagnetization curve has a slope between 1.00 and 1.25.
The permanent magnet preferably has a temperature coefficient xcex2 of intrinsic coercivity of between 0.30%/xc2x0 C. and xe2x88x920.30%xc2x0 C. The permanent magnet also preferably has a temperature coefficient of residual induction Br of between +0.02%/xc2x0 C. to xe2x88x920.040%/xc2x0 C.
The permanent magnet also has a small cellular structure, having a cell size of preferably xe2x89xa6100 nm.
A preferred composition of the permanent magnet is one in which the effective z is between about 6.5 and about 8.0, w is between about 0.50 and about 0.85, v is between 0.0 and about 0.35, x is between about 0.05 and about 0.20, and y is between about 0.01 and about 0.05. In one preferred embodiment, the permanent magnet comprises from between about 22.5% and about 35.0% by weight effective Sm (samarium), between about 42% and about 65% by weight Co (cobalt), between 0.0% and about 25% by weight Fe (iron), between about 2.0% and about 17.0% by weight Cu (copper), and between about 1.0% and about 5.0% by weight Zr (zirconium). In another preferred embodiment, the magnet. comprises from between about 23.5% and about 28.0% by weight effective Sm, from between about 50% and about 60% by weight Co, from between about 4.0% and about 16% by weight Fe, from between about 7.0% and about 12% by weight Cu, and from between about 2.0% and about 4.0% by weight T, where T is as defined above.
In another embodiment of the invention, the permanent magnet comprises about 24.7% by weight effective Sm, about 57.8% by weight Co, about 7.0% by weight Fe, about 7.1% by weight Cu, and about 3.4% by weight of a mixture of Zr and Nb. In yet another embodiment, the permanent magnet comprises about 26% by weight effective Sm, about 59.5% by weight Co, about 3.3% by weight Fe, about 7.6% by weight Cu, and about 3.6% by weight of a mixture of Zr and Nb. In yet another embodiment, the magnet preferably comprises about 26% by weight effective Sm, about 61.0% by weight Co, about 1.0% by weight Fe, about 8.2% by weight Cu, and about 3.8% by weight of a mixture of Zr and Nb.
In another alternative embodiment of the invention, a permanent magnet is provided having the general formula RE(CowFevCuxTy)z is provided, where RE is a rare earth element selected from the group consisting of Sm, Gd, Pr, Nd, Dy, Ce, Ho, Er, La, Y, Tb, and mixtures thereof, T is a transition metal selected from the group consisting of Zr, Hf, Ti, Mn, Cr, Nb, Mo, W, V, Ni, Ta, and mixtures thereof, the sum of w, v, x and y is 1; and z has a value between about 6.5 and 8.0.
The permanent magnet of the present invention is preferably prepared by increasing the cobalt content as the operating temperature increases. The cobalt content (w) of the magnet is preferably determined by the formula w=0.5332+0.0004935xc2x7TM. The magnet may be prepared so as to have a maximum operating temperature TM of between about 340xc2x0 C. to about 700xc2x0 C.
Accordingly, it is a feature of the present invention to provide a permanent magnet which retains its magnetic properties and exhibits a linear extrinsic demagnetization curve at operating temperatures up to 700xc2x0 C. Other features and advantages of the invention will be apparent from the following description, the accompanying drawings, and the appended claims.